Disclosed in the prior art are adhesive compositions based on solutions of elastomeric polymers in polymerizable vinyl monomers. The compositions usually exhibit reasonably good storage stability and cure rapidy to strong, tough bonds. The compositions cure in the presence of air at room temperature, usually with the aid of various polymerization catalysts; e.g., free-radical generators, initiators, promoters, and accelerators.
Examples of such adhesive compositions can be found in U.S. Pat. Nos. 3,810,407, 3,962,372, 4,112,013, 4,118,436, 3,994,764, 3,832,274, and 4,138,449. Adhesive compositions formulated from these patents have several shortcomings which make them ineffective in applications which involve extreme environmental conditions.
One major disadvantage is their inability to maintain their structural properties such as tensile shear strength, T-peel and impact after significant exposure to high temperature aging. The chlorosulfonated polyethylene, polychloroprene and polyurethane rubber based adhesive compositions are limited, with respect to their maximum useful service, to temperatures less than 110.degree. C. All of these adhesives require the addition of reasonably large amounts of epoxy resins to improve high temperature resistance. However, high temperature improvements in this manner are not always substantial enough to meet the requirements of many high temperature applications, and often are merely temporary.
A particular disadvantage of the poly(butadiene) based adhesive compositions is that they are soluble in relatively few monomers. Recent legislation regarding hazardous organic solvents has focused on the need to replace high volatile monomers with low volatility, high boiling-point monomers. The prior art rubbers (elastomers) are generally not sufficiently soluble in low volatility methyl methacrylate monomers to allow a useful product to be formulated.
Furthermore, unsaturated diene elastomers, notably the poly (styrene-co-butadiene) and poly(butadiene) polymers, show poor resistance to the individual or combined effect of molecular oxidation and ozonation, both at elevated and normal temperatures. These formulations also exhibit poor resistance to sunlight exposure, UV radiation, and various adverse environmental aging conditions, such as extremes in temperature combined with moisture or solvents. As a result of the cross-linking reactions of these butadiene polymers, embrittlement of the polymer generally occurs. To overcome extreme embrittlement, antioxidants, antiozonants, UV absorbers, oils and waxes often must be used as protective agents or to provide protective surfaces. Yet, such additions of materials would be expected to influence the free-radical catalyzation reactions which could result in delayed or retarded cure speed. Other disadvantages of the diene elastomers include the poor resistance to oil, solvent, acid and alkali materials and the inability to crystalize upon distortion, all of which act to decrease the structural integrity of the polymer and hence decrease the bond strength. The polychloroprene-type elastomers cure to give rigid, brittle bonds that cannot withstand substantial impact shock or peeling forces.
It is evident that there is a need for an adhesive composition which when cured, provides excellent bond strengths and exhibits toughness and resistance to environmental conditions, particularly high temperature, for extended periods of time.
The adhesive compositions of this invention are designed to overcome these traditional problems of the prior art.